Silicone rubber has excellent characteristics of weatherability, electric characteristics, a low compression set resistance, a heat resistance, a cold resistance and the like and has thus wide use in various fields including electric instruments, motor vehicles, construction, medical services and foods. For instance, examples of use applications include key pads used for rubber contact points of remote controllers, typewriters, word processors, computer terminals, musical instruments and the like, building gaskets, anti-vibration rubbers for audio devices and the like, vehicle parts such as connector seals and spark plug boots, packings for compact disks used in computers, and molds for breads and cakes. The demand for silicone rubber is now increasing more and more, and the development of silicone rubber having excellent characteristics has been demanded.
Of these, the key pad material has been in wide use such as for keyboards of cell phones and personal computers. The characteristics required for the key pad materials are those wherein when a key is pressed, a load variation is small. Usually, when a molded key is repeatedly pressed, the load of the key lowers as the number of the repetitions increases. A less lower of peak load leads to better key characteristics, and a material showing such a load characteristic as mentioned above is excellent as a key pad material.
Silicone rubber has been in wide use as such a key pad material. In JP-A 2001-164111 (Patent Document 1), a silicone rubber composition for key pad has been proposed.
In recent years, however, the strain exerted on the key becomes greater in association with a more complicated shape of a molded key. Moreover, the miniaturization of a recently employed device leads to an increasing number of shapes that need a greater strain exerted on material. Therefore, recent severe requirements would not be adequately satisfied with respect to dynamic fatigue durability.
In JP-A 2009-275158 (Patent Document 2), there has been proposed a silicone rubber composition suited as a key pad having excellent dynamic fatigue durability (keying durability) wherein there is used a phosphoric acid ester having an alkyl group, part of which is substituted with chlorine. Such a composition is not favorable because the keying durability is not satisfactory and a making device undergoes corrosion. To solve this problem, JP-A 2011-105782 (Patent Document 3) has proposed a silicone rubber composition suitable for use as a key pad having excellent dynamic fatigue durability, in which an organodisilazane having an alkenyl group in the molecule and a fatty acid ester and/or a fatty alcohol ester is used. However, the organodisilazane having an alkenyl group in the molecule is expensive, thus being poor in economy. Further, the use of the organodisilazane is unfavorable in that the nitrogen-containing compounds derived from the unreacted organodisilazane and the resulting reaction side product serve as a catalytic poison, so that addition vulcanization is disenabled, or a cured product undergoes yellowing.